We waited until DD was 8.
She's a little bit on the sensitive side, and we had some sadness and tears with books 6 and 7. She also likes to read right before she goes to bed, so I was a little concerned about bad dreams.

She read them all in one summer. Once they get started with this series, they go go go. She would not have wanted to stop with the early books.

Six might be a little young to handle the later books, but I guess it really depends on the child.